1. Field of the Invention
The present invention relates to a charging device for uniformly charging an object to be charged such as a photosensitive body and an image forming apparatus such as an electrophotographic copying apparatus having the charging device.
2. Description of the Related Art
For example, in an electrophotographic copying apparatus, after a photosensitive body serving as an image carrier is uniformly charged, an image is exposed on the photosensitive body, thereby forming a latent image corresponding to an original. In addition, a toner is electrostatically attracted to the latent image on the photosensitive body to form a toner image, and this toner image is transferred to paper.
As a conventional charging device for uniformly charging a photosensitive body, a corotron charging device has been popularly used.
This corotron charging device comprises a charging wire (also called a corona discharging wire) and a shield case having a U-shaped sectional structure which surrounds the charging wire and has an opening portion opposing the photosensitive body serving as an object to be charged.
A high voltage is applied to the charging wire to cause the charging wire to generate a corona current, and this current flows onto the photosensitive body, thereby supplying charges onto the surface of the photosensitive body.
On the other hand, in an electrophotographic copying apparatus, an increase in copy speed is strongly demanded. For this reason, as the image carrier of a high-speed copy machine, a highly photosensitive material such as arsenic selenide is used.
In addition, the highly photosensitive material such as arsenic selenide sets the surface of a photosensitive body in a mirror-surface state and eliminates disadvantages caused by a large surface roughness to prolong the service life of the photosensitive body. That is, this material prevents a discharging effect caused by a pin-like projection or a charging fault caused by entering paper dust or a toner into recessed portions in the surface of the photosensitive body.
A conventional charging device has an arrangement in which one charging wire is arranged at the central position of the opening width of the shield case. Another conventional charging device has an arrangement in which two charging wires are arranged at symmetrical positions with respect to the central position of the opening width of a shield case.
In addition, a charging device having an arrangement in which a charging wire is offset to the downstream side of a shield case as described in Jpn. Pat. Appln. KOKAI Publication No. 61-210370 such that a voltage applied to the charging wire is set to be low to obtain a predetermined charging amount, has been developed.
with the above arrangement, the peak of a corona current (flow-in current) flowing from the opening portion of the shield case onto the rotating photosensitive body is set at or near the central position of the opening width of the shield case or a position offset to the downstream side with respect to the central position of the shield case.
However, when the charging wire is arranged at the central position of the shield case or the position offset to the downstream side in the rotational direction of the photosensitive body, the peak of charging is set at the central position of the shield case or the position offset to the downstream side in the rotational direction of the photosensitive body. In this case, a charging irregularity disadvantageously occurs.
A case wherein a charging irregularity occurs when a drum having a rough surface which is not subjected to a mirror-surface process is used will be considered. For example, on a drum having a surface which has a surface roughness R.sub.Z .apprxeq.0.8 and on which fine projecting and recessed portions are present, since both the projecting and recessed portions are charged, an amount of charge per unit area is large. For this reason, even when some charging irregularity occurs, an amount of charge per unit area does not largely vary, and a degree of attraction of the toner is rarely influenced.
In contrast to this, when a planished drum having, e.g., a surface roughness R.sub.Z .apprxeq.0.2 is used, since no projecting and recessed portions are present on the surface of the photosensitive body, an amount of charge per unit area is small. For this reason, when a charging irregularity occurs in an area, an amount of charge in the area largely varies, and an area on which no charges are present is formed on the surface of the photosensitive body. Therefore, the toner is not attracted to this area, thereby forming pitmarks in the resultant image. In particular, this phenomenon becomes conspicuous when a halftone image is output, and the phenomenon is a problem to be solved when a planished drum is used.